A metal nanoparticle having a particle diameter of about 1 to 200 nm has been paid attention in various properties such as an electrical property, a magnetic property, or a catalyst property, and has been expected with applications in various fields.
A metal colloid obtained by dispersing a nanoparticle of a precious metal ouch as gold in a dispersion medium, has a photo absorption property called “plasmon absorption” generated by a plasma vibration of an electron, so it has a color characteristic of the respective metal elements.
A gold colloid for example has the above described plasmon absorption near 520 nm and expresses a go-called wine red color.
The above described gold colloid for example is applied to a coating material in which a solution containing the colloid is coated and heated to form a gold plate, a pregnancy test agent obtained by bonding an antibody, and biosensing such as a genetic tooting for detecting a DNA having a specified nucleotide sequence. And, by coating and drying the gold colloid, a fine line as a thin film or a wire of gold can be formed.
The precious metal colloid such as the gold colloid described above has a weak affinity between a dispersed phase of a precious metal fine-particle and the dispersion medium of water, so it is unstable and will be prone to cause a flocculation when an electrolyte presents.
Then, for preventing the flocculation, it is demanded to add a large amount of a protection agent having a function for stabilizing the precious metal colloid.
However, when the precious metal colloid containing the large amount of the protection agent made of mainly polymer, is coated and dried as described above in forming a conductive thin film, the protection agent without a conductivity obstructs the conductivity. Therefore, the film is demanded to perform a high temperature treatment to decompose an organic matter. However, due to such high temperature treatment, a dimensional precision of the conductive thin film will be lowered.
While the application of the gold colloid as described above has been spread, a method for producing the gold colloid in simple and low cost has been demanded.
Japanese Unexamined patent Application (Kokai) No. 2001-192712 for example discloses a method for reducing a center metal by a reduction elimination of ligand from a quaternary ammonium salt metal complex compound to form a metal nanoparticle.
However, there is disadvantage that it is demanded to compose a specified compound such as the quaternary ammonium salt metal complex compound, which is difficult to produce easily.
Japanese Unexamined patent Application (Kokai) No. 11-76800 discloses a method for dissolving a metal compound in a solvent, adding a polymer pigment dispersion agent, adding an alkanolamine or other amines as a reduction agent, and reducing the metal to form the metal colloid protected by the polymer pigment dispersion agent.
However, there are disadvantages that the production is not easy due to using alkanolamine or other strong toxicity agents and a reduction of a production cost is difficult due to using the polymer pigment dispersion agent or other materials which is hard to obtain.
Japanese Unexamined patent Application (Kokai) No. 11-80647 discloses a method for using a polymer pigment dispersion agent having a specified structure of 2,000 to 1,000,000 in average molecular weight and an alcohol as the reduction agent to form a precious metal or copper colloid used to a colorant having a high chroma.
An example in Japanese Unexamined patent Application (Kokai) No. 10-66861 discloses a method for using silver nitrate as a metal compound, sodium citrate as a dispersion agent, and ferrous sulfate or tannic acid a the reduction agent and mixing them under stirring in 2,000 to 6,000 rpm to form a silver colloid.
By the above two methods, however, a thin film having a high conductivity is not able to be obtained.
An example in Japanese Unexamined patent Application (Kokai) No. 2002-245854 discloses a method for using a compound having an amino group and a carboxyl group, such as an amino acid, as the protection agent and reducing the precious metal by tannic acid.
It is disclosed that a precious metal colloid obtained by the above method has a stability enabling a conductive ink. The precious metal colloid however is gradually deposited, so it is not suitable for a long-term storage.
In the above way, a combination of well-known start materials suffers from disadvantages that a desired high conductivity is not obtained in forming the thin film, a reaction is carried out at a production condition having a lot of restrictions for an uniform dispersion, and the conductivity is sacrificed when the polymer pigment dispersion agent is used to obtain a high dispersability.